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.. meta::
  :description: API reference for cuda::counting_semaphore, a scoped counting semaphore for constraining concurrent access between GPU and CPU threads in libhipcxx for HIP and CUDA.
  :keywords: libhipcxx, ROCm, HIP, C++, counting_semaphore, semaphore, concurrent access, thread scope, synchronization

.. _libcudacxx-extended-api-synchronization-counting-semaphore:

``cuda::counting_semaphore``
============================

This page documents ``cuda::counting_semaphore``, a scoped counting semaphore for constraining concurrent access between GPU and CPU threads.

Defined in header ``<cuda/semaphore>``:

.. code:: cuda

   template <cuda::thread_scope Scope,
             cuda::std::ptrdiff_t LeastMaxValue = /* implementation-defined */>
   class cuda::counting_semaphore;

The class template ``cuda::counting_semaphore`` is an extended form of `cuda::std::counting_semaphore <https://en.cppreference.com/w/cpp/thread/counting_semaphore>`_
that takes an additional :ref:`cuda::thread_scope <libcudacxx-extended-api-memory-model-thread-scopes>` argument.
``cuda::counting_semaphore`` has the same interface and semantics as
`cuda::std::counting_semaphore <https://en.cppreference.com/w/cpp/thread/counting_semaphore>`_.

Concurrency Restrictions
------------------------

An object of type ``cuda::counting_semaphore`` or ``cuda::std::counting_semaphore``, shall not be accessed concurrently
by CPU and GPU threads unless:

- it is in unified memory and the `concurrentManagedAccess property <https://docs.nvidia.com/cuda/cuda-runtime-api/structcudaDeviceProp.html#structcudaDeviceProp_116f9619ccc85e93bc456b8c69c80e78b>`_
  is 1, or
- it is in CPU memory and the `hostNativeAtomicSupported property <https://docs.nvidia.com/cuda/cuda-runtime-api/structcudaDeviceProp.html#structcudaDeviceProp_1ef82fd7d1d0413c7d6f33287e5b6306f>`_
  is 1.

Note, for objects of scopes other than ``cuda::thread_scope_system`` this is a data-race, and therefore also prohibited
regardless of memory characteristics.

Under CUDA Compute Capability 6 (Pascal) or prior, an object of type ``cuda::counting_semaphore`` or
``cuda::std::counting_semaphore`` may not be used.

Implementation-Defined Behavior
-------------------------------

For each :ref:`cuda::thread_scope <libcudacxx-extended-api-memory-model-thread-scopes>` ``S`` and least maximum value
``V``, ``cuda::counting_semaphore<S, V>::max()`` is as follows:

.. list-table::
   :widths: 50 50
   :header-rows: 0

   * - :ref:`cuda::thread_scope <libcudacxx-extended-api-memory-model-thread-scopes>` ``S``
     - ``cuda::binary_semaphore<S>::max()``
   * - Any thread scope
     - ``cuda::std::numeric_limits<cuda::std::ptrdiff_t>::max()``

Example
-------

.. code:: cuda

   #include <cuda/semaphore>

   __global__ void example_kernel() {
     // This semaphore is suitable for all threads in the system.
     cuda::counting_semaphore<cuda::thread_scope_system> a;

     // This semaphore has the same type as the previous one (`a`).
     cuda::std::counting_semaphore<> b;

     // This semaphore is suitable for all threads on the current processor (e.g. GPU).
     cuda::counting_semaphore<cuda::thread_scope_device> c;

     // This semaphore is suitable for all threads in the same thread block.
     cuda::counting_semaphore<cuda::thread_scope_block> d;
   }

`See it on Godbolt <https://godbolt.org/z/3YrjjTvG6>`_